UCSC scientists part of team decoding gamma-ray burst mystery

Scientists have pieced together the key elements of a gamma-ray burst, from star death to dramatic black hole birth, thanks to a March 29 explosion considered the "Rosetta stone" of such bursts.

The results are described in the June 19 issue of Nature, in an article coauthored by Stan Woosley, professor and chair of astronomy and astrophysics at the University of California, Santa Cruz.

The telling March 29 burst in the constellation Leo, one of the brightest and closest on record, reveals for the first time that a gamma-ray burst and a supernova--the two most energetic explosions known in the universe--occur essentially simultaneously, a quick and powerful one-two punch.

The burst was detected by NASA's High-Energy Transient Explorer (HETE) and observed in detail with the European Southern Observatory's Very Large Telescope (VLT) at the Paranal Observatory in Chile.

"The March 29 burst changes everything," said Woosley. "With this missing link established, we know for certain that at least some gamma-ray bursts are produced when black holes, or perhaps very unusual neutron stars, are born inside massive stars. We can apply this knowledge to other burst observations." The research team said that just as the Rosetta stone helped us understand an ancient language, this burst will serve as a tool to decode other gamma-ray bursts.

Woosley and his graduate student, Weiqun Zhang, created computer simulations of a gamma-ray burst using one of the fastest unclassified computers in the world, at Lawrence Berkeley National Laboratory. Using 128 computer processors simultaneously, Woosley said the simulations took about two weeks--or about 25,000 processor hours. Woosley is director of the Center for Supernova Research, funded by the Department of Energy's Scientific Discovery through Advanced Computing (SciDAC) program. The other partner institutions are the University of Arizona and the Los Alamos and Lawrence Livermore National Laboratories.

A supernova is the explosion of a star at least eight times as massive as the Sun. When such stars deplete their nuclear fuel, they no longer have the energy to support their mass. Their cores implode, forming either a neutron star or (if there is enough mass) a black hole. The surface layers of the star blast outward, becoming a billion times as luminous as the Sun.

Scientists have suspected gamma-ray bursts and supernovae were related, but they have had little observational evidence, until March 29.

"We've been waiting for this one for a long, long time," said Jens Hjorth, University of Copenhagen, lead author of the Nature article, one of three related "letters" appearing in the issue. "The March 29 burst contains all the missing information. It was created through the core collapse of a massive star."

The research team said that the Rosetta stone burst also provides a lower limit on how energetic gamma-ray bursts truly are and rules out most theories concerning the origin of "long bursts," lasting longer than two seconds.

Gamma-ray bursts temporarily outshine the entire universe in gamma-ray light, packing the energy of over 100 million billion suns.

"It is as bright as if two thousand Earth masses were abruptly turned to energy in the form of gamma rays," said Woosley. "Any life within the path of the burst inside of several hundred light-years would have been obliterated."

Yet these explosions are fleeting--lasting only seconds to minutes--and occur randomly from all directions on the sky, making them difficult to study.

GRB 030329, named after its detection date, occurred relatively close, approximately 2 billion light years away (at redshift 0.1685). The burst lasted over 30 seconds. ("Short bursts" are less than 2 seconds long.) GRB 030329 is among the 0.2 percent brightest bursts ever recorded. Its afterglow lingered for weeks in lower-energy X-ray and visible light.

With the VLT, Hjorth and his colleagues uncovered evidence in the afterglow of a massive, rapidly expanding supernova shell, called a hypernova, at the same position and created at the same time as the afterglow. The following scenario emerged:

Thousands of years prior to this explosion, a very massive star, running out of fuel, let loose much of its outer envelope, transforming itself into a bluish Wolf-Rayet star. The Wolf-Rayet star--containing about 10 solar masses worth of helium, oxygen, and heavier elements--rapidly depleted its fuel, triggering the Type Ic supernova/gamma-ray burst event. The core collapsed, without the star's outer part knowing. A black hole formed inside surrounded by a disk of accreting matter, and, within a few seconds, launched a jet of matter away from the black hole that ultimately made the gamma-ray burst.

The jet passed through the outer shell of the star and, in conjunction with vigorous winds of newly forged radioactive nickel-56 blowing off the disk inside, shattered the star. This shattering represents the supernova event.

Meanwhile, collisions among pieces of the jet moving at different velocities, all very close to light speed, created the gamma-ray burst. This "collapsar" model, introduced by Woosley in 1993, best explains the observation of GRB 030329, as opposed to the "supranova" and "merging neutron star" models.

In previous gamma-ray bursts, scientists had found evidence of iron in the afterglow light, a signature of a star explosion. Also, the location of a supernova occurring in 1998, named SN1998bw, appeared to be in the same vicinity as a gamma-ray burst. The data was inconclusive, however, and many scientists remained skeptical of the association.

"Supernova 1998bw whetted our appetite," said coauthor, Chryssa Kouveliotou of the NASA Marshall Space Flight Center in Huntsville, Alabama. "But it took five more years before we could confidently say we found the smoking gun that nailed the association between gamma-ray bursts and supernovae."

"This does not mean that the gamma-ray burst mystery is solved," Woosley said. "We are confident that long bursts involve a core collapse, probably creating a black hole. We have convinced most skeptics. We cannot reach any conclusion yet, however, on what causes short gamma-ray bursts."

Short bursts might be caused by neutron star mergers. A NASA-led international satellite named Swift, to be launched in January 2004, will "swiftly" locate gamma-ray bursts and may capture short-burst afterglows, which have yet to be detected.

The VLT is the world's most advanced optical telescope, comprising four 8.2-meter reflecting Unit Telescopes and, in the future, four moving 1.8-meter Auxiliary Telescopes for interferometry. HETE was built by MIT as a mission of opportunity under the NASA Explorer Program, with collaboration among U.S. universities, Los Alamos National Laboratory, and scientists and organizations in Brazil, France, India, Italy, and Japan.